NOTES FOR MICROECONOMICS

by
Prof. Nicholas Economides
Stern School of Business
Fall 2011

Copyright  Nicholas Economides

1
MICROECONOMICS is about
1. Buying decisions of the individual
2. Buying and selling decisions of the firm
3. The determination of prices and in markets
4. The quantity, quality and variety of products
5. Profits
6. Consumers’ satisfaction

There are two sides in a market for a good
DEMAND SUPPLY
Created by Consumers Created by firms
Each consumer maximizes Each firm maximizes its
satisfaction (“utility”) profits
------------------------------- ------------------------------
 CONSUMPTION THEORY  PRODUCTION THEORY

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We will first study consumption and later production. In the third part of the
course we will take the “demand” schedule from the consumption analysis and
the “supply” schedule from the production analysis and put them together in a
market. The price, and the quantity exchanged will be determined in the
market. We will also discuss the performance and efficiency of markets.

A. CONSUMPTION ANALYSIS UNDER CERTAINTY
1. Goods are products or services that consumers or businesses desire.
Examples: a book, a telephone call, insurance coverage. Goods may be
directly desired by consumers or may contribute to the production of other
goods that are desired by consumers. For example a machine used in the
production of cars is desirable because it is useful in the production of cars,
although it has no direct value to a consumer. Bads are products or services that
consumers desire less of. Examples: garbage, pollution, some telephone calls.
Clearly, a good for one consumer could be a bad for another.
2. If possible, each consumer would consume a very large (infinite) amount of
each good. But, each individual is constrained by his/her ability to pay for these
goods. The limitation of total funds available to an individual defines the

3
budget constraint. Therefore a consumer has to maximize his/her satisfaction
while not spending more than he/she has, i.e., without violating the budget
constraint.
3. We are interested to find the best choice for a consumer that has a limited
amount of funds. We accomplish this in three steps. At the first step, we define
the available choices taking into account the limitation of funds. At the second
step, we discuss the desires/wants of the consumer. At the third step, we find
the optimal choice for the consumer by putting together the information we
gathered in the previous two steps.
STEP 1: We first analyze the available choices to a consumer that possesses
limited funds. Suppose there are only two goods, X and Y, and they are sold at
prices p
x
and p
y
per unit respectively. If a consumer buys x units of good X
and y units of good Y, she spends xp
x
on good X, and yp
y
on good Y. Total
expenditure is
E = xp
x
+ yp
y
.
The pair (x, y) is called a (consumption) basket or (consumption) bundle.
If the consumer has a total amount of money I (income) her total expenditure
cannot exceed I, i.e.,

4
xp
x
+ yp
y
 I.

This is called the budget constraint. The set of available (x, y) combinations is
called the budget set. See Figure 1.

Figure 1
For example, X is apples sold at $1 per pound, Y is oranges sold at $0.5 each,
and the consumer has I = $3 to spend. Then the basket (1, 4) (i.e., 1 pound of
apples and 4 oranges) costs 1 + 4(.5) = $3, and therefore is in the budget set.

5
Note that since basket (1, 4) available to the consumer, so are baskets where she
buys less of each or less of both of the goods, such as (0, 4), (1, 3), (1, 2), etc.,
since they cost less.
4. The budget constraint can be represented in the X-Y space. There are two
cases, either the consumer spends all her income and xp
x
+ yp
y
= I, or the
consumer has some left-over income and xp
x
+ yp
y
< I. We are primarily
interested in the case where all income is spent. Note that and xp
x
+ yp
y
= I is a
straight line in the X-Y space. It is called the budget line. Its slope is -p
x
/p
y
. If
the consumer spends all the money in good X then she buys I/p
x
units of this
good. This is the maximum amount of X that she can buy. It defines the most
extreme point of the triangle on the X-axis. If all money is spend on good Y, it
will buy I/p
y
units of good Y. This is the most extreme point of the triangle on
the y-axis.
All bundles (x, y) where the consumer does not exhaust all income are below
the budget line. The budget set contains all the points in the shaded triangle,
including its boundary lines.
STEP 2: We now describe the preferences of the consumers.
5. In a comparison of any two bundles, A = (x
A
, y
A
) and B = (x
B
, y
B
), an

6
individual should be able to say either
(i) “I prefer A to B”; or
(ii) “I prefer B to A”; or
(iii) “I am indifferent between A and B”, i.e., “I like equally A and B”.
This property of preferences is called completeness. Essentially the consumer
is not allowed to say “I don’t know” or “I am not sure.”
The second property of preferences is transitivity. If a person states, “I prefer
A to B,” as well as “I prefer B to C,” then he/she also has to prefer A to C. This
assumption says that preferences are consistent, so that comparisons between
bundles A and C are consistent with comparisons between bundles A and B and
between B and C.
Transitivity in indifference means that a person who says, “I am indifferent
between A and B,” as well as “I am indifferent between B and C,” also has to
be indifferent between A and C.
The third assumption on preferences is that “more is better”.
6. Since “more is better”, if bundle A = (x
A
, y
A
) has more of both goods than
bundle B = (x
B
, y
B
), i.e., if x
A
> x
B
and y
A
> y
B
, then clearly a consumer will
prefer A to B. Similarly if bundle A has less of both goods than bundle B, then

7
a consumer will prefer B to A. However, if bundle A has more of X but less of
Y than B, the comparison is not obvious. The consumer may prefer A to B, or
prefer B to A, or be indifferent between A and B.
One can create a collection of all the bundles, A, B, C, D, ..., such that a
particular consumer is indifferent between any two of them. The line in X-Y
space that connects the points in this collection {A, B, C, D, ...} is called an
indifference curve, I
1
(Figure 2). Of course, the same consumer typically has
many indifference curves. For example, he has both A-B-C-D and E-F-G-H as
indifference curves, and he prefers any bundle on E-F-G-H to any on A-B-C-D.
In general, there is an indifference curve through any point in X-Y space.
Since “more is better,” an indifference curve cannot have a positive slope.
Indifference curves have a negative slope, and in special cases zero slope. An
indifference curve defines the substitution between goods X and Y that is
acceptable in the mind of the consumer. As we move towards the Southeast
along a typical indifference curve the consumer receives more X and less Y,
while she declares that she is equally well off.

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Figure 2
7. Typically indifference curves are convex. This means that starting with two
bundles, A, B, which the consumer the consumer likes equally and are therefore
on the same indifference curve, she prefers C, the average of two extreme
bundles, rather than either of them. Bundle C has the average quantities of
bundles A and B in X and Y (Figure 3).

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Figure 3
8. Special cases of indifference curves. If the goods are perfect substitutes,
the indifference curves have a constant slope, i.e., are straight lines. This means
that the substitution between good X and good Y is constant, irrespective of the
point on the indifference curve. Example, X is nickels, Y is dimes (Figure 4).
If the goods are perfect complements, the consumer combines the goods in a
fixed proportion. Then, indifference curves are L-shaped. Examples: X is left

Figure 5
9. A consumer can be thought of as assigning a level of satisfaction (or utility)
to each bundle, utility of bundle (x, y) is U(x, y). Then, all bundles on the same
indifference curve give the same level of satisfaction (utility). For example, in
our earlier figure, the level of satisfaction of a consumer may be 2 at any point
on indifference curve EFGH, and the level of satisfaction may be 1 at any point

12
on indifference curve ABCD.
10. The marginal rate of substitution, is the rate at which a consumer is
willing to trade x for y. It is the slope of an indifference curve, MRS = Δy/Δx.
In general, the MRS varies along an indifference curve, that is, the MRS is in
general different when the starting bundle of a potential trade changes. For
perfect substitutes, the MRS is constant. Note that the marginal rate of
substitution (MRS) of consumer Z depends on individual preferences as
expressed by the indifference curves. It does not depend on the market or the
prices that may prevail in the market.
11. An additional unit of good X increases the level of satisfaction of a
consumer by the marginal utility of X, MU
x
. Similarly, an additional unit of Y
increases the level of satisfaction of a consumer by the marginal utility of Y,
MU
y
. The marginal rate of substitution is equal to the ratio of the marginal
utilities,
MRS = Δy/Δx = - MU
x
/MU
y

Why? In Figure 6, consider a move from bundle A to bundle B on the same
indifference curve. It can be broken into a vertical piece (change in Y) and a
horizontal piece (change in X) by defining bundle C.

13

Figure 6
MOVE CHANGE IN UTILITY
A to C (MU
y
)(Δy)
C to B (MU
x
)(Δx)
────────────────────────────────────────────
TOTAL, A to B (MU
x
)(Δx) + (MU
y
)(Δy) = 0
Total change in utility between A and B is zero because A and B are on the

14
same indifference curve. Rearranging the terms in this we derive the slope of
the indifference curve,
(MU
y
)(Δy) = -(MU
x
)(Δx)  MRS = Δy/Δx = - MU
x
/MU
y
.
12. For a convex indifference curve, its slope goes from high on the left to low
on the right. This means that, as the consumer has more Y, she is willing to
give up less and less in X in exchange for acquiring equal amounts of Y. Her
indifference curves exhibit diminishing marginal rate of substitution.
STEP 3: We now find the optimal choice of the consumer by combining the
analysis of her preferences with her available choices.
13. Given convex and smooth indifference curves, the consumer maximizes
utility at a point A, where the slope of the indifference curve (MRS) is equal to
the slope of the budget constraint. At the chosen point A we have tangency of
the indifference curve and the budget constraint line (Figure 7),
p
x
/p
y
= MRS = MU
x
/MU
y
, i.e., MU
x
/p
x
= MU
y
/p
y
.
This means that the consumer receives equal satisfaction for the last dollar spent
in each good. The quantity of X that consumer Z chooses at A is called his
demand for X. The demand of consumer Z varies as prices and income
change. We denote it with x
*
(p
x
, p
y
, I). Similarly the demand of this consumer

15
for good Y is y
*
(p
x
, p
y
, I).
Figure 7
14. Tangency can fail at the optimal point
(1) If the indifference curves are not smooth, for example if they have a kink,
as in the case of perfect complements (Figure 8);
(2) If the optimal point is at a corner of the budget set (Figure 9).

16

Figure 8

17

Figure 9
15. Changes in income. As income expands the consumer changes his levels
of consumption. If more of X is consumed, then X is a normal good (Figure
10). Example: high quality clothes. If less of X is consumed, then X is an
inferior good (Figure 11). Example: low quality food, subway tokens. The
consumption bundles A, B, C, as income increases are on the income
expansion path.

18

Figure 10

19

Figure 11
16. Changes in prices. As the price of X decreases, there is a natural tendency
to consume more of the good that became cheaper. But, at the same time,
because of the price decrease, the consumer suddenly finds herself more
wealthy. She can buy the old bundle, and still have left-over money. We can
separate these two effects on the consumption of X as the substitution effect

20
and the income effect. There is a natural tendency to buy more of the cheaper
good. This is measured, roughly speaking, by the substitution effect. The
extra money left-over unspent after the price decrease may be spent on X or on
Y. The increase or decrease in the consumption of X resulting from the

Figure 12
spending of the left-over money is measured, roughly speaking, by the income

21
effect. Note that a consumer may not like good X as much when she is richer,
and could decrease its consumption as her income increases.
17. Consider a price decrease that makes the consumer move from A to B. We
break it into the change from A to C (substitution effect) and the change from C
to B (income effect). We isolate the substitution effect by taking away from the
consumer enough money to put her at the same level of satisfaction as before
the price change. In Figure 12, the line through C has the same slope as the one
through B, and is tangent to the indifference curve through A.
18. The direction of the substitution effect is always opposite to the price
change. If the good is normal, the income effect is in the same direction. The
two effects reinforce each other. Therefore, if a good is normal, the demand
curve (that shows how much would be sold at different prices) as slopes
downward. This is called the law of demand. A typical demand curve is
shown in Figure 12a. This far we have used the notation x for quantity and p
x

for price. For most of the remaining of the course we will use the notation Q
for quantity and P for price. Notice that there are two alternative ways to
interpret what a demand curve shows. First, the demand curve shows how
many units people are willing to buy at any particular price: Q(P). Second the

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demand curve shows what price would be fetched if a certain number of units
of output were offered at the market: P(Q).

Figure 12a
19. If the good is inferior, the income effect goes in the opposite direction of
the substitution effect (Figure 13). Typically, even for inferior goods, the
demand slopes downward, because the income effect is smaller than the
substitution effect. If the good is very strongly inferior (in very rare cases), the

23
income effect could be bigger than the substitution effect, so that the total effect
is opposite to the substitution effect, and the demand slopes upward. Then the
good is called a Giffen good.

24
In the first, only good X (say gasoline) is taxed, so that its price goes from p
x
to
p
x
+ t, where t is the tax per gallon. In the second wealth, I, is taxed. Suppose
that both schemes raise the same amount of total tax. Then the second scheme
leaves consumers better off (i.e., on a higher indifference curve).

Figure 14
21. Elasticities measure the responsiveness of quantities traded to prices or

25
income. Price elasticity measures the percentage change in quantity as a
response to a percentage change in price. The (own) price elasticity of
demand is
e = (ΔQ/Q)/(Δp/p) = (ΔQ/Δp)(p/q).
Note that e < 0, and that the elasticity is not the slope of the demand curve. For
example, for a linear demand curve Q = a – bP, the slope is ΔQ/Δp = -b
(constant) but the elasticity of demand is –bp/q which varies as the quantity (or
price) changes.
If e < -1, i.e., e > 1, the demand is elastic, i.e., highly responsive to changes
in price. Typical for non-necessities, goods you do not have to buy, luxuries.
If e > -1, i.e., e < 1, the demand is inelastic, i.e., not responsive to changes in
price. Typical for necessities, goods that you have to buy.
If e = -1, i.e., e = 1, the demand is called uni-elastic.
Generally, a demand is more elastic if the product has close substitutes.
Demand for an individual brand is more elastic than market demand. Typically,
elasticity of long run demand is higher than elasticity of short run demand
because of wider availability of substitutes.
22. The income elasticity of demand measures the responsiveness of the

26
quantity demanded on income changes.
e
I
= (ΔQ/Q)/(ΔI/I) = (ΔQ/ΔI)(I/Q).
If e
I
> 0, the good is normal. If e
I
< 0, the good is inferior. Most goods are
normal. However, goods you buy when you have low income may be inferior.
23. The cross elasticity of demand measures the responsiveness of the
demand for good X on price changes of another good, Y.
e
x,py
= (Δx/x)/(Δp
y
/p
y
).
If e
x,py
> 0, x and y are substitutes (say Diet Coke and Diet Pepsi). If e
x,py
< 0, x
and y are complements (say computers and printers).
24. If all units are sold at the same price, the consumers who are willing to buy
at a high price benefit from the existence of consumers who are willing to pay
only a low price. All units are sold at a price equal to the willingness to pay for
the last unit. The difference between what a consumer is willing to pay and
what he actually pays is called consumers surplus.

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Figure 15
25. The total willingness to pay up to Q units is the area under the demand up
to Q units, A(Q). The actual expenditure is E(Q) = QP(Q). The difference is
consumers’ surplus,
CS(Q) = A(Q) - E(Q).

In Figure 15, expenditure E(Q) is double-shaded, and consumers' surplus CS(Q)
is single-shaded. A(Q), the total willingness covers both shaded areas.

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B. CONSUMPTION ANALYSIS UNDER UNCERTAINTY
26. Risk and Uncertainty.
Example 1: Consider the choice between receiving $10 with certainty and
receiving $8 or $12 with probability 1/2 each. Note that both alternatives have
the same expected value of $10. The utility of the first alternative is U(10), and
the utility of the second alternative is U(8)/2 + U(12)/2. Remember, both
alternatives have the same expected monetary value of $10, but only the first
one guarantees this amount with certainty. A risk-averse person will prefer the
first (riskless) alternative. This means that for a risk averse person,
U(10) > U(8)/2 + U(12)/2.

Note that, for a risk-averse person, the utility of wealth is concave. See Figure
16. This means that the marginal utility of wealth (the utility of the last dollar)
is decreasing with wealth.

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Figure 16
A risk-lover will prefer the second (risky) alternative, i.e., for him
U(10) < U(8)/2 + U(12)/2.

Note that, for a risk-lover, the utility of wealth is convex. See Figure 17. This
means that the marginal utility of wealth (the utility of the last dollar) is
increasing with wealth.

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Figure 17
A risk-neutral person is indifferent between the two alternatives,
U(10) = U(8)/2 + U(12)/2.

For a risk-neutral person, the utility of wealth is a straight line. This means that
the marginal utility of wealth (the utility of the last dollar) is constant for any
level of wealth.
We define the certainty-equivalent of an uncertain situation as the amount of

31
money x that, if received with certainty, is considered equally desirable as the
uncertain situation, i.e.,
U(x) = U(8)/2 + U(12)/2.
For a risk-averse person the certainty equivalent must be less than 10, x < 10.
For a risk-neutral person x = 10, and for a risk-lover x > 10.
Example 2: Suppose that a consumer has utility function
U(W) = √W = W
1/2
,
where W is her wealth. First check that the consumer is risk averse (either by
showing that √W is concave or by showing that he prefers $10 rather than the
$8 or $12 lottery above). Suppose that this consumer with probability 1/200 (=
5/1000) has $10,000, and with probability 99/200 (= 995/1000) has $1,000,000.
How much will she be willing to accept with certainty in return for this
uncertain situation?
Solution: Facing the uncertain situation, her utility is
.995*√1,000,000 + .005*√10,000 = .995*1,000 + .005*100 =995+0.5= 995.5

The certainty-equivalent is $x such that √x = 995.5, i.e., x = $(995.5)
2
=
$991,020.25. The consumer is willing to accept $991,020.25 with certainty in
return for her uncertain situation. Note that because the consumer is risk averse

(if no “bad” event occurs), but his wealth is diminished to W
B
with probability
prob
B
(if the “bad” event occurs). Since these are the only two eventualities,
prob
G
+ prob
B
=1 must be true. The bad event creates a loss of L = W
G
- W
B
.
In general, an insurance company is willing to take away an uncertain situation
and give in return to the consumer the same amount of money in both the
“good” and “bad” eventualities. To be able to guarantee the same wealth even
if the bad event happens, the insurance company collects a fee, called the
premium. Thus, the insurance contract takes away from the consumer the
uncertain world {W
G
with probability prob
G
, W
B
with probability prob
B
} and
replaces it with a certainty world {W, W}. The remaining question is how does
W relate to W
G
and W
B
. To start with, it must be that W is between the high
and low wealth W
G
< W < W
B
(why?), but we need to find more specifically
what W is acceptable to the consumer.
Example 3: A consumer with utility function U(W) = √W has wealth from two

33
sources. He has $10,000 in cash, and a house worth $990,000. Suppose that
the house burns down completely with probability 0.005 (that is 0.5% or 5 in a
thousand). The house lot is assumed to be of no value. How much is the
consumer willing to pay for full insurance coverage?
Notice that the consumer has wealth $1,000,000 with probability 0.995, and
$10,000 with probability 0.005, so her uncertain situation is exactly as in
example 2. We have found above that the certainty equivalent of this uncertain
situation for this consumer is $991,020.25. Therefore she is willing to accept
any insurance contract that would leave her with wealth of at least $991,020.25
(hopefully higher). The insurance contract that is just acceptable is the one that
leaves her with exactly $991,020.25. In insurance terms, this is a contract with
coverage of $990,000 and premium P
max
. To find the maximum premium the
consumer is willing to pay, P
max
, we reason as follows. If the house does not
burn down, the consumer has wealth of $991,020.25 = $1,000,000 (original
wealth) – P
max
(premium). If the house burns down, the consumer has again
wealth $991,020.25 = $10,000 (cash) + $990,000 (payment from the insurance
company) – P
max
(premium). Therefore the maximum premium acceptable to
the consumer is P
max
= $1,000,000 - $991,020.25 = $8,979.75.

34
Of course, the consumer is willing to accept full coverage of $990,000 for a
lower premium than P
max
. One such premium is the one that corresponds to an
actuarially fair insurance contract that guarantees with certainty the dollar
expected value of the wealth of the uncertain situation it replaces. (In example 1
this expected value is $10.) In example 2, the expected value of the wealth of
the uncertain situation is (as we have calculated above)
.995*1,000,000 + .005*10,000 = 995,000 + 50 = $995,050.
Therefore the actuarially fair premium is P
act.fair
= $1,000,000 - $995,050 =
$4,950. If the insurance company is risk neutral, charging the actuarially fair
premium allows the company to just break even. Any additional premium
above the actuarially fair is profit to the risk neutral insurance company.
Therefore the insurance company can have a profit of up to P
max
- P
act.fair
=
$8979.75 - $4,950 = $4029.75. In summary, a risk neutral insurance company
will not charge a premium below $4,950 and the consumer will not accept a
premium above $8,979.75. The premium will be between these two numbers
and will depend on competition among insurance companies.
C. PRODUCTION AND COSTS
28. Output Q is produced from many inputs including labor man-hours L,

35
capital (machine hours) K, land use, and other factors. For the purposes of this
course, we will assume a single variable factor of production, labor L. The
relationship between labor and output is summarized by the production function
Q = f(L). We define the marginal productivity of labor as MP
L
= df/dL. We
also expect that marginal productivity MP
L
to be decreasing at high usage
levels of the particular input (L). See Figure 18. We also define the average
productivity
AP
L
= Q/L = f(L)/L.

Since marginal productivity eventually (i.e., with high use of the input)
decreases, it will eventually drive the average productivity down too.
29. The cost of production of Q units is the cost $wL of the input L required to
produce quantity Q = f(L), where $w is the price of labor. For example if Q =
f(L) = 10L, then the amount L required to produce Q units is Q/10, and
therefore the cost to produce Q is C(Q) = wQ/10. Similarly, if Q = f(L) = 10√L,
the amount L required to produce Q units is (Q/10)
2
and therefore the cost to
produce Q is C(Q) = w(Q/10)
2
.

38
ATC(q) = F/q + AVC(q).
Incremental (marginal) cost: MC(q) = C'(q) = dC/dq = V'(q) = dV/dq.
Incremental or marginal cost is the cost of production of an extra unit of q.
Since extra production does not affect fixed cost, the increase in the total cost is
the same as the increase in the variable cost, MC(q) = dC/dq = dV/dq.
32. We will examine three types of cost functions. The first type of cost
functions exhibits constant returns to scale. Every unit of output costs the same
to produce. Marginal cost is equal to average cost, AC(q) = MC(q). See Figure
19. Goods with constant returns to scale are typically hand-made where the
same amount of labor needs to be put in for every unit.
33. The second type of cost functions exhibits increasing returns to scale for
every level of production. Marginal cost is the same for every unit, but there is
an additional fixed or setup cost. See Figure 20. Many manufactured goods
require a fixed cost for the creation of a design and thereafter the incremental
costs are constant. Examples: software, microchips.
34. The third type of cost functions exhibits increasing returns to scale for
small production levels and decreasing returns to scale for large production
levels. See Figure 21. Example: traditional manufacturing.

42
36. Typically, profits are negative for small q because of positive setup costs
that cannot be recovered at low sales. Profits increase in q, reach a maximum,
and then decrease. See Figure 23. In this course, we will assume that the firm
tries to maximize profits. At the quantity level q
*
that maximizes profits, the
slope of Π(q), dΠ/dq (incremental or marginal profit), is zero. At any q,
dΠ/dq = R'(q) - C'(q) = MR(q) - MC(q).

This condition is necessary for profit maximization irrespective of the
organization of the industry and of the nature of competition. As we will see
below, the nature and extent of competition in a market changes MR but usually
has no significant influence on MC.
D. MARKET STRUCTURE
37. By definition, in perfect competition, no firm has any influence on the
market price. Therefore, each unit of output is sold at the same price, p.
Therefore, for a competitive firm, MR(q) = p. Each firm perceives a horizontal

See Figure 24. If the price falls below minimum average total cost, the firm
closes down. Therefore the supply function of a competitive firm in the long
run is its marginal cost curve above minimum ATC.

44
run, a firm cannot recover its fixed costs (by closing down). Therefore the
relevant cost function in the short run is average variable cost, AVC. The firm’s
supply function in the short run is MC above minimum AVC. In the short run, it
supplies zero if p < min AVC. This means that there is a range of market
prices, between min ATC and min AVC, such that the firm will
produce a positive quantity in the short run, but will shut down in the long run.
Profits are the shaded box in Figure 24,
Π(q
*
) = R(q
*
) - C(q
*
) = q
*
[p - AC(q
*
)].

Figure 25 shows the market supply with free entry of firms.

45

Figure 25

46

Figure 26
39. Producers surplus is the difference between the revenue of the firm and its
variable costs,
PS(q) = R(q) - V(q).
Variable costs, V(q) can be represented by the area under the marginal cost
curve (Figure 26). V(q) is the minimum revenue a firm is willing to accept in

47
the short run to produce q.
Total surplus (Figure 27) is the sum of consumers and producers surplus,
TS(q) = CS(q) + PS(q).
Since consumers surplus is the area under the demand minus revenue,
CS(q) = A(q) - R(q),
total surplus is
TS(q) = A(q) - R(q) + R(q) - V(q) = A(q) - V(q).
It is maximized at the quantity q
c
where the marginal cost curve intersects the
demand curve.

48

Figure 27

49

Figure 28
40. If the market is not perfectly competitive, each firm faces a downward
sloping demand. Since revenue is
R(q) = qp(q),

where │ε│ is the elasticity of the demand faced by the firm. That is, marginal

50
revenue is intimately related to elasticity. Note that when the demand is
horizontal, or perfectly elastic, │ε│ = ∞, the formula for marginal revenue
gives MR = p(1 - 0) = p. For a downward sloping demand, see Figure 30, and
the table below.

Figure 29

51

Demand curve Marginal revenue
Elastic, │ε│ > 1 MR > 0
Unit-elastic, │ε│ = 1 MR = 0
Inelastic, │ε│ < 1 MR < 0
41. Monopoly. A monopolist produces q
m
that solves MR(q
m
) = MC(q
m
).
Because MR(q) = p(1 - 1/│ε│) < p, for the same level of quantity, the marginal
revenue to a monopolist is lower than to a competitive firm. As a result, a
monopolist produces a lower level of output than a competitive firm,
q
m
< q
c
,
where q
c
is at the intersection of marginal revenue and marginal cost. The
lower quantity produced by the monopolist corresponds to a higher price,
p
m
> p
c
.
Monopoly is inefficient and it creates a surplus loss (Figure 32). This is
because monopoly production is lower than the surplus-maximizing production,
q
m
< q
c
. Each unit q between q
m
and q
c
costs less to produce that the price that
someone is willing to pay for it as seen from the demand curve, i.e., MC(q) <
p(q). Therefore, it is socially beneficial for these units to be produced, but the

52
monopolist does not produce them since this is not in his private interest. The
inefficiency due to monopoly is called dead weight loss and is measured in $
terms by the triangle enclosed by the demand curve, the MC curve and line q
m
.

Figure 30

53

Figure 31
42. Price discrimination occurs when different units of the same product are
sold at different prices to different consumers, or even to the same consumer.
Examples: 1. A unit of electricity (a KiloWattHour, KWH) is sold at a lower
price to a manufacturer than to residential customer. 2. Your Con Edison Bill
has a lower price for each of the first 240 KWH than for units above 240. 3.
Japanese car manufacturers have been accused of selling in the US market at a
lower price than in their home market. 4. Residential local telephone service

54

Figure 32

costs less than business local telephone service. 5. Doctors sometimes charge
lower prices to poorer patients. 6. Single product loyalty discounts for high
levels of consumption, as in the airlines frequent flyer programs.
There are two essential requirements for price discrimination to work
effectively. They are sorting and no arbitrage. The seller must be able to sort
the buyers in groups according to their willingness to pay for the good.
Otherwise, he does not know to whom to offer the high or low price. The seller

55
must also make sure that the buyers who were able to buy at a low price do not
go around and sell what they bought to other buyers who are only offered a high
price by the seller. Such activity is called arbitrage and would result in a single
price in the market. For example, price discrimination can work well in the
market for medical services, since it is impossible to transfer the service from
one person to the other. However, price discrimination is very hard to work in
the stock market. The low price recipients would sell the stocks to the others,
and arbitrage would equalize the price.
43. In perfect price discrimination each consumer is offered a price equal to his
willingness to pay for the good. Each buyer's consumer surplus is zero.
Producers appropriate the whole surplus. In imperfect price discrimination
consumers are split in groups according to willingness to pay, but the sorting is
not perfect. For example, the local TV cable monopoly can charge a different
price to “uptown” customers than to downtown customers.
Another example of price discrimination is a two-part tariff. For example, your
Verizon bill includes a fixed fee which does not depend on the number of calls,
on top of the price you pay for each phone call. Disneyland used to charge a
fixed fee at the entrance plus a fee per ride.

56
Bundling is a special case of price discrimination, in which two goods are sold
together (as a bundle) at a lower price than separately. For example, a computer
system (composed of a CPU, a video monitor, a hard drive, a keyboard, and
Windows) is sold at a lower price than the sum of the a la carte prices of the
individual components. Bundled components do not have to be related (e.g.,
Polaroid-TWA). Additionally, in a more complex bundling contract, a
company may offer a loyalty discount on a bundle of products attempting to
leverage its power in one market to other markets. For example: a contract
might say: if you buy 90% of all your needs from company A, you get a
discount.
44. Perfect Price Discrimination. Under perfect price discrimination, each
unit is sold at the willingness to pay for that unit of the person who is buying it.
Therefore the revenue of the perfectly price discriminating monopolist is the
whole area under the demand up to his level of production q,
R(q) = A(q).

57

Figure 34

His marginal revenue is the revenue generated by the sale of the last unit, i.e.,
the price of this last unit q on the demand curve,
MR(q) = p(q).

58
q
c
. Remember, in this case consumers surplus is zero, and profits are equal to
total surplus. Despite this distributional inequality, perfect price discrimination
is efficient because it maximizes total surplus.
45. Game theory. Games describe situations where there is potential for
conflict and for cooperation. Many business situations, as well as many other
social interactions have both of these such features.
Example 1: Company X would like to be the only seller of a product (a
monopolist). The existence of a competing firm Y hurts the profits of firm X.
Firms X and Y could cooperate, reduce total production, and increase profits.
Or they could compete, produce a high quantity and realize small profits. What
will they do?
Example 2: Bank 1 competes with bank 2 for customers. Many of their
customers use Automated Teller Machines (ATMs). Suppose that each bank
has a network of its own ATM machines which are currently available only to
its customers. Should bank 1 allow the customers of the other bank to use its
ATMs? Should bank 1 ask for reciprocity?
Example 3: Computer manufacturer 1 has a cost advantage in the production of
network “cards” (interfaces) of type 1. Similarly manufacturer 2 has an

59
advantage in network “cards” of type 2. If they end up producing cards of
different types, their profits will be low. However, each firm makes higher
profits when it produces the “card” on which it has a cost advantage. Will they
produce “cards” of different types? Of the same type? Which type?
A game in extensive form is defined by a set of players, i = 1, ..., n, a game
tree, information sets, outcomes, and payoffs. The game tree defines the
sequence and availability of moves in every decision node. Each decision
node is identified with the player that decides at that point. We assume there is
only a finite number of possible moves at every node. Each branch of the tree
ends at an event that we call an outcome. The utility associated with the
outcome for every player we call his payoff. Information sets contain one or
more nodes. They show the extent of knowledge of a player about his position
in the tree. A player only knows that he is in an information set, which may
contain more than one nodes. Information sets allow a game of simultaneous
moves to be described by a game tree, despite the sequential nature of game
trees. A game where each information set contains only one point is called a
game of perfect information. (Otherwise it is of imperfect information.) For
example, in the “Incumbent-Entrant” game (Figure 35), at every point, each

60
player knows all the moves that have happened up to that point. All the
information sets contain only a single decision node, and the game is of perfect
information. In the “Simultaneous Incumbent-Entrant” game (Figure 36),
player I is not sure of player E's decision. He only knows that he is at one of the
two positions included in his information set. It is as if players I and E move
simultaneously. This is a game of imperfect information. Note that this small
change in the information sets of player I makes a huge difference in what the
game represents -- a simultaneous or a sequential decision process.
When the utility functions associated with the outcomes are known to both
players, the game is of complete information. Otherwise it is of incomplete
information. You may not know the opponents' utility function. For example,
in a price war game, you may not know the value to the opponent firm (or the
opponent manager) of a certain loss that you can inflict on them.
46. A game in normal form is a summary of the game in extensive form.
This is facilitated by the use of strategies. A strategy for player i defines a
move for this player for every situation where player i might have to make a

61

Figure 35

move in the game. A strategy of player i is denoted by s
i
, and it belongs in
the set of available strategies of player i, S
i
. By its nature, a strategy can be
very complicated and long. For example, a strategy for white in chess would

62

Figure 36

have to specify the opening move, the second move conditional on the 20
alternative first moves of the black, the third move conditional on the many (at
least 20) alternative second moves of the black, and so on. The advantage of
using strategies is that, once each player has chosen a strategy, the outcome
(and the corresponding payoffs) are immediately specified. Thus, the analysis

Strategies for Player 1: Enter, Stay out
Strategies for Player 2: High Q, Low Q

Example 2: “Prisoners' Dilemma Game”
Player 2
Strategies silence Talk

Player 1
Silence (5, 5) (0, 6)
Talk (6, 0) (2, 2)

Strategies for player 1: Silence, Talk.
Strategies for player 2: silence, talk.

64

Figure 37
47. Non-cooperative equilibrium. A pair of strategies (s
1
, s
2
) is a non-
cooperative equilibrium if and only if each player has no incentive to change
his strategy provided that the opponent does not change his strategy. No player
has an incentive to unilaterally deviate from an equilibrium position.
Looking at the Prisoners' Dilemma, we see that if player 2 plays “silence” and is

65
expected to continue playing this strategy, player 1 prefers to play “Talk”, since
he makes 6 instead of 5 in the payoff. Since player 1 wants to deviate from it,
(Silence, silence) is not a non-cooperative equilibrium. If player 2 plays “talk”
and is expected to continue playing it, player 1 prefers to play “Talk”, since he
makes 2 instead of 0 in the payoff. Therefore (Talk, silence) is not a non-
cooperative equilibrium. Finally, given that player 1 plays “Talk”, player 2
prefers to play “talk” since he gets 2 instead of 0. Since both players prefer not
to deviate from the strategy they play in (Talk, talk) if the opponent does not
deviate from his strategy in (Talk, talk), this is a non-cooperative equilibrium.
In the “simultaneous Incumbent-Entrant” game, if E enters, the incumbent
prefers to play L because his payoff is 8 rather than 6. If the incumbent plays
L, the entrant chooses to play Enter, because he prefers 8 to 0. Therefore no
player has an incentive to deviate from (Enter, Low Q), and it is a non-
cooperative equilibrium. In the same game, if the entrant chooses to stay out,
the incumbent replies by producing a high quantity (H) because 18 is better than
9. And, if the incumbent produces a high Q, the entrant prefers to stay out
because 0 is better than -3. Therefore no player has an incentive to deviate
from (Stay out, High Q), and it is also a non-cooperative equilibrium. Note

66
that there are two equilibria in this game.
To find the equilibrium in the original sequential “Incumbent-Entrant” game of
Figure 35, note that if player E enters, player I prefers Low Q, and ends up at
(8, 8). If E stays out, I prefers High Q, and ends up at (0, 18). Seeing this, E
chooses to enter because in this way he realizes a profit of 8 rather than of 0.
Therefore the non-cooperative equilibrium is at (Enter, Low Q), and both
firms realize a profit of 8. Note that the other equilibrium of the simultaneous
game was eliminated.
48. Dominant strategies. In some games, no matter what strategy player 1
plays, there is a single strategy that maximizes the payoff of player 2. For
example, in the Prisoners’ Dilemma if player 1 plays “Silence”, it is better for
player 2 to play “talk”; and, if player 1 plays “Talk”, it is better for player 2 to
play “talk” again. Then “talk” is a dominant strategy for player 2. In the same
game, note that “Talk” is a dominant strategy for player 1, because he prefers it
no matter what player 2 plays. In a game such as this, where both players have
a dominant strategy, there is an equilibrium in dominant strategies, where
each player plays his dominant strategy. An equilibrium in dominant strategies
is necessarily a non-cooperative equilibrium. (Why? Make sure you

67
understand that no player wants to unilaterally deviate from a dominant strategy
equilibrium.)
49. There are games with no equilibrium in dominant strategies. For
example, in the simultaneous incumbent-entrant game, the entrant prefers to
stay out if the incumbent plays “H”. However, the entrant prefers to enter if the
incumbent plays “L”. Since the entrant would choose a different strategy
depending on what the incumbent does, the entrant does not have a dominant
strategy. (Similarly, check that the incumbent does not have a dominant
strategy.) Therefore in the simultaneous incumbent-entrant game there is no
equilibrium in dominant strategies.
50. Strategic Trade Policy. In many industries firms face U-shaped average
cost functions with minimum efficient scale of significant size. Then the
industrial policy or the trade policy of a country can have very significant
impact on the entry of firms and the existence of an industry in a country.
Consider an entry game between Boeing and Airbus with the following profit
matrix.
Entry in Airplane Manufacturing Without Subsidies
Player 2 (Airbus)

This game has two non-cooperative equilibria. In each one of them, one firm
enters and the other does not. However, if Airbus received a subsidy of 200
from France the profit matrix gets transformed into:
Entry in Airplane Manufacturing with Airbus Subsidy Only
Player 2 (Airbus)
Strategies enter don’t enter

Now the “enter” strategy becomes dominant for Airbus, and the only non-
cooperative equilibrium is when Airbus enters and Boeing stays out. If the US
were to subsidize Boeing by 200 in the absence of an Airbus subsidy, in the
new matrix the “enter” strategy would now be dominant for Boeing and the
only non-cooperative equilibrium would have Boeing entering and Airbus

69
staying out. Of course, each country could each subsidize its company.
Entry in Airplane Manufacturing with Two Subsidies
Player 2 (Airbus)
Strategies enter don’t enter

Then both firms have a dominant strategy to enter. The public is the loser in
this case, because the market is forced to have more firms than is efficient.
51. Best replies. Player 1's best reply to strategy s
2
of player 2 is defined as
the best strategy that player 1 can play against strategy s
2
of player 2. For
example, in the simultaneous incumbent-entrant game, the best reply of the
entrant to the incumbent playing “High Q” is “Stay out”. Similarly, the best
reply of the entrant to the incumbent playing “Low Q” is “Enter”. From the
point of view of the incumbent, his best reply to the entrant's choice of “Enter”
is “Low Q”, and his best reply to the entrant's choice of “Stay out” is “High Q”.
Notice that at a non-cooperative equilibrium both players play their best replies
to the strategy of the opponent. For example, at (Stay out, High Q), as we saw,

70
“Stay out” is a best reply to “High Q”, and “High Q” is a best reply to “Stay
out”. This is no coincidence. At the non-cooperative equilibrium no player has
an incentive to deviate from the strategy he plays. This means that his strategy
is the best among all the available ones, as a reply to the choice of the opponent.
This is just another way of saying that the player chooses the best reply
strategy. Therefore at equilibrium each player plays a best reply strategy.
This suggests that we can find equilibria by finding first the first reply
strategies. We do this in the oligopoly games that we discuss next.
52. Oligopoly. When few firms are interacting in a market, they have to take
into account the effects of each one's actions on the others. We use game
theory to analyze these strategic situations.
The simplest oligopoly model is due to Augustin Cournot (1838). There is a
single homogeneous good with demand function p(Q). Two competing firms, i
= 1, 2, produce q
i
each. Q = q
1
+ q
2
. The profit function of firm 1 is
Π
1
= q
1
p(q
1
+ q
2
) - C
1
(q
1
).

In game theoretic terms, this is a game where each player uses the quantity he
produces as his strategy. According to Cournot, each firm assumes that (at
equilibrium) its actions have no influence on the actions of the opponent.

71
Therefore we seek the non-cooperative equilibrium of the game.
Figure 38
By assuming that firm 1 has no influence on the output of firm 2, Cournot
assumed that the residual demand facing firm 1 is just a leftward shift by q
2
of
the industry demand p(Q) (Figure 38). Firm 1 is a monopolist on this residual
demand. For example, firm 2 produces q
2
= 100 units. The remaining demand

72
is drawn as D
R
. Since firm 1 is a monopolist on D
R
, we draw his marginal
revenue curve, MR
R
, and find his optimal output q
1
b
at the intersection of MC
and MR
R
. This is the output that maximizes profits for firm

Figure 39
1 provided that firm 2 produces q
2
= 100. This means that q
1
b
is the best reply
of firm 1 to q
2
= 100. Similarly we can define the best reply of player 1 to

Therefore, in comparison to the Cournot equilibrium, in monopoly price and
profits are higher, while quantity is lower.
55. The following table summarizes the comparison between different
industrial structures, also seen in Figure 41.

77
56. Price Leadership, Cartels.
Here the leader sets the price p and the follower(s) accept the price and act as if
they were in perfect competition. This is a common market behavior of cartels
such as OPEC. They fix the price for the good and it is accepted by others in
the industry outside the cartel.

Figure 42

78
The follower(s) are price-takers. Let their supply be S
f
(p). If the industry
demand is D(p), the leader faces the residual demand D
R
(p) = D(p) - S
f
(p). The
leader acts as a monopolist on the residual demand. Note that if the leader and
the follower have the same marginal costs, since because the leader has to
restrict his output (to achieve a high price) and the follower doesn't, the leader
will have lower profits than the follower. This explains the incentive of a cartel
member to leave the cartel. While inside the cartel, each member has to restrict
production so that the cartel is able set a high price. If a firm moved out of the
cartel, and the price remained the same, then the defector would produce more
and make higher profits. The catch is that usually, when a firm leaves the
cartel, the ability of the cartel to set the price is diminished. Therefore higher
profits for the defector are not guaranteed, since he can sell more, but at a lower
price. See Figure 42.
57. Monopolistic competition. We started by studying perfect competition
where no firm has any influence over the market price. We then studied its
diametrically opposite market structure, i.e., monopoly, where a single firm sets
the market price. We proceeded to study oligopoly where few firms interact.
Using game theory we analyzed the strategic interactions

79
among the few market participants. There is an extra category of market
structure that we haven't discussed yet. It is monopolistic competition and it
falls between oligopoly and perfect competition. There are a number of
competing firms, say above ten, but less than twenty five. In monopolistic
competition firms make zero profits. This distinguishes monopolistic
competition from oligopoly. At the same time, in monopolistic competition
Figure 43

80
each firm faces a downward-sloping demand function. Therefore each firm has
some influence over price. This distinguishes monopolistic competition from
perfect competition. In monopolistic competition there are strategic interactions
among firms, but they are small. The following table summarizes the
similarities and differences of the various market structures.
Perfect
Competition

MRP
K
is the productivity (in $) of capital. Therefore it is the demand for
capital. Depending on the conditions in the output market, marginal revenue,
MR, may or may not equal to the output price p. For example, a monopolist
has MR < p. If two firms, a monopolist and a competitive firm, use the same
production process and have the same physical productivity of capital MPP
K
,
for the same level of output the monopolist will have a lower marginal revenue
product of capital, MRP
K
, than the competitive firm. Since MRP
K
is the
demand for capital, a monopolist will demand and employ less labor than a
competitive firm of the same technology.
In the input market, price-taking behavior by the buyers of inputs means than a
firm can buy all the quantity it wants at a single price, v. This means that each
firm that buys in the input market, sees a horizontal the supply curve for the
input.

82
Figure 44

83
59. Supply of Labor
An individual consumes leisure (H) and “consumption goods” (C). His utility
function is U(C, H). Both H and C are goods. Suppose that the initial
endowments are H = 24 (hours in the day) and C = C
o
. The time not spent on
leisure is spend as labor L,
L = 24 - H.

The slope of the budget constraint is -w, the wage rate. Maximizing utility
U(C, H) subject to the budget constraint above he chooses (C
*
, H
*
). This means
that he supplies L
*
= 24 - H
*
.
When the wage rate increases to w' > w, there are two effects on the consumer.
First, he has an incentive to work more, because each hour of work is worth

84

Figure 45
more in consumption goods. Second, he is more rich, even if he works exactly
the same amount of hours as before. Because he is more rich, he would like to
consume more leisure and work less. Therefore the two effects of an wage
increase on the supply of labor go in opposite directions. The total effect can go
in either direction. This means that an increase in wage can lead to smaller or
larger supply of labor. In Figure 46, the increase in wage to w' leads to a

85

Figure 46
reduction in the supply of labor. Such a phenomenon is called the backward-
bending supply curve for labor. See Figure 46.

86
60. Supply of capital and intertemporal choice. The derivation of the
supply and demand for capital is a simple application of utility maximization
with endowments. Suppose that there are only two periods in the world, 1 and
2 . In period 1 there is only one good c
1
, and in period 2 there is only good c
2
.
A consumer has income m
1
in period 1 and m
2
in period 2. You should think of
c
1
and m
1
as expressed in dollars of period 1; similarly, c
2
and m
2
are expressed
in dollars of period 2. The consumer can consume more than m
1
in period 1 by
borrowing against his income of period 2. The consumer could also consume
less than m
1
in period 1 by lending some of his income. Suppose that for every
dollar borrowed in period 1, the consumer has to pay (1+r) dollars in period 2.
Then r is called the interest rate. Suppose that the consumer can borrow and
lend at the same rate. If the consumer borrows (c
1
- m
1
) in period 1 he has to
pay back (c
1
- m
1
)(1 + r) in period 2. Since there is no later period, this amount
has to equal the difference between income and consumption in period 2, i.e.,
m
2
- c
2
= (c
1
- m
1
)(1 + r), i.e., c
2
= m
2
+ (m
1
- c
1
)(1 + r).

This is the budget constraint of the consumer. See Figure 47. The consumer's
preferences between consumption in period 1 and consumption in period 2 are
expressed through his indifference curves. Maximizing satisfaction, the

87
consumer picks point A where there is tangency between the indifference curve
and the budget constraint. In Figure 47, the consumer spends c
1
*
in period 1,
and borrows c
1
*
- m
1
. He spends only c
2
*
in period 2. He uses the remaining of
his income in period 2, m
2
- c
2
*
to pay his debt, which has grown with interest to
(c
1
*
- m
1
)(1 + r). We know from the budget constraint that these two amounts
are equal.
Figure 47
61. Interest rates. As we have seen, given the interest rate, some consumers
decide to borrow and some consumers decide to lend money. Summing the

88
amounts the consumers are willing to borrow in period 1 at different interest
rates we derive the demand for funds in period 1. Similarly, summing the
amounts the consumers are willing to lend in period 1 at different interest rates
we derive the supply of funds in period 1. The intersection of supply and
demand determines the market interest rate.
62. Present value. A project x can be thought of as a stream of incomes in
consecutive time periods x = (x
0
, x
1
, ..., x
n
). Any of these numbers can be zero,
positive or negative. An easy way to evaluate projects is by computing present
values. The present value of next year’s return is x
1
/(1+r
1
), where r
1
is the
interest rate between periods 0 and 1. The present value in period 0 of period
2's return is x
2
/[(1+r
1
)(1+r
2
)], where r
2
is the interest rate between period 1 and
2. Similarly, the present value in period 0 of period n’s return is
x
n
/[(1+r
1
)(1+r
2
)...(1+r
n
)], where r
n
is the interest rate between period n-1 and n.
Note that in general the interest rate between period 0 and 1 will be different
than the interest rate between 1 and 2, and so on. For the project x = (x
0
, x
1
, ...,
x
n
) that has returns in all periods, the present value (PV) is defined as
PV(x) =x
0
+ x
1
/(1+r
1
) + x
2
/[(1+r
1
)(1+r
2
)] +...+x
n
/[(1+r
1
)...(1+r
n
)].
In adopting projects, a manager can use present values, and adopt the project

89
that has the higher PV.
63. Bonds and relationship to interest rates. A bond is an obligation by
the US government or other entities. Depending on the issuer, bonds have
different risks. The risk of a bond is the risk of default and no final payment of
the obligation that the bond represents. The least risky bonds are the ones
issued by the US Treasury. Their low risk is reflected in their low return. High
risk bonds (junk bonds) have higher returns.
64. We now consider the determination of the price of bonds and their
relationship with interest rates. This discussion is limited to riskless (US
Treasury) bonds. A typical US Treasury Bond (or Note) B has a face value F,
and a coupon C. A bond with expiration after n periods pays F after n periods;
it also pays C at the end of each period between now and its expiration. From
the point of view of an individual that buys it, the bond can be considered as a
“project”. The stream of returns for the bond are (C, C, ..., C, F), where C
appears in the first n-1 positions, and F in the nth position. What is the
present value of the bond? Using the formula we have,
PV(B) = C + C/(1+r
1
) + C/[(1+r
1
)(1+r
2
)] + ... + C/[(1+r
1
)...(1+r
n-1
)]

+ F/[(1+r
1
)...(1+r
n
)].

90
Note that the present value of a bond is inversely related to the interest rates r
1
,
r
2
, ..., r
n
.
65. The US Treasury issues bonds and notes of many durations. For
simplicity, suppose a period is a year, and there are bonds of duration of one
year B
1
, two years B
2
, and so on to n years B
n
. Assume also that irrespective of
length, all bonds have the same face value F and same coupon C. Their present
values are easy to calculate using the general formula. For example, the present
value of the one-year bond is,
PV(B
1
) = C + F/(1 + r
1
).

Now consider that these bonds are auctioned today. They must fetch exactly
their present value. Given the interest rate r
1
, there is a unique present value for
bond B
1
. Conversely, if we know the price that the bond fetched in the auction
(and which is its present value) we can use the formula above to find the interest
rate r
1
,
PV(B
1
) = C + F/(1 + r
1
)  r
1
= F/[PV(B
1
) - C] - 1.

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When it is auctioned today, its price is PV(B
2
). Thus, we know the value of the
Left-Hand-Side of the formula. In the Right-Hand-Side of this formula, we
already know r
1
from the analysis of the one-year bond. The value of the
interest rate r
2
follows immediately. We can repeat this analysis step by step for
bonds of longer duration. In this way, the auction prices of US treasury bonds
reveal the interest rates of varying durations.
66. Application to the EU financial crisis: bonds exchange in the face of
uncertainty when participants have varying expectations. Consider the
secondary market for Greek sovereign bonds that mature a year from now that
have a coupon C and face value F (which are known to all), and suppose that,
assuming no default, the interest rate for Greek bonds is r
1
. For example,
assume that r
1
= 5% = 0.05. Once we take into account the expectations of
default by market participants, the market interest rate will be different than r
1
.
We will construct the demand for the Greek bond and find the market interest
rate. To make things concrete, suppose there are €40 billion of such bonds in
circulation (total supply), a vertical line at 40 billion.
67. Suppose we are absolutely sure that Greece will pay the coupon C, but
there is uncertainty about whether Greece will pay the full face value after a

Even a small haircut implies a high interest rate (discount rate). A 10% haircut
results in a 16.7% interest rate (from 5% interest rate without a haircut). A 20%
haircut implies a 31.25% interest rate (close to the present interest rate for
Greek bonds). But if we observe a high implied interest rate, does this mean that
everyone believes there will be a haircut?

93
68. Suppose investors are divided on their expectations of whether Greece
will impose a haircut. Example: half of the investors expect no haircut and the
other half expects a 20% haircut. This implies that the demand curve consists of
two lines: a horizontal line at price PV(1) for quantities 0 to 20 billion, and a
second horizontal at price PV(0.8) for quantities 20 to 40 billion . You can
easily see that the equilibrium price for the bond is PV(0.8), which brings the
discount rate from 5% to 31.25%. This means that it is sufficient for some part
of the market to expect a haircut for the interest rate to soar.

F. GAINS FROM TRADE AND COMPETITIVE EQUILIBRIUM
69. Gains from trade. Suppose there are two goods, X, Y, and two
individuals (traders), S, and J, endowed with (x
A
S
, y
A
S
) and (x
A
J
, y
A
J
) respectively.
We construct the Edgeworth box of dimensions x
A
J
+ x
A
S
and y
A
J
+ y
A
S
. A
point, such as A, in the Edgeworth box is an allocation. The traders J and S
can reach any other point, such as B, by trading an amount of X for an amount
of Y. See Figure 48.
70. We can draw the indifference curves for person J and person S through
point A. In general, these indifference curves intersect (i.e., are not tangent),

94
and therefore they create a lens of Pareto superior allocations to A. Any point,
such as C, in the lens is preferred to A by both persons. Starting from A, both
persons are willing to trade to reach C.

Figure 48
71. A point, such as D, where the two indifference curves passing through it
are tangent, cannot be improved upon through trade. There is no lens to move
to. Such a point is called Pareto optimal. The collection of Pareto optimal

95
points is called the contract curve. The portion of the contract curve in the
lens is called the core. It consists of all allocations that have two properties. (1)
They are superior to A for both persons (and therefore are in the lens); and (2)
They are Pareto optimal (and therefore are on the contract curve).

Figure 49
72. One common way to trade is with constant prices. An individual with

The right-hand-side is the value of her endowment. The left-hand side is the
value of the consumed bundle (x, y). In Figure 49, the consumer chooses
bundle B which has less y and more x. Therefore, starting with bundle A, the
consumer sells y and buys x to reach bundle B.
73. A competitive equilibrium has the following properties: (1) Each trader
maximizes utility for given prices p
x
, p
y
; (2) The total quantity demanded by
consumers is equal to the total supply.
In an Edgeworth box, the budget line for both traders passes through the point
of initial endowments, A, and has slope -p
x
/p
y
. Suppose that both traders
maximize their utility by picking up the same point, B, on the budget line. See
Figure 51. We know that any point in the Edgeworth box signifies a feasible
distribution of total resources. In other words, the total quantity demanded of
each good at B is equal to the total resources,
x
B
J
+ x
B
S
= x
A
J
+ x
A
S
and y
B
J
+ y
B
S
= y
A
J
+ y
A
S
.

Therefore, if traders pick up the same point, B, then we have a competitive
equilibrium.

97
The significance of the competitive equilibrium is that traders can reach a
Pareto optimal point without meeting or bargaining. Traders just observe
prices. If all markets clear, a Pareto optimal point has been reached. In the
diagram, for markets to clear, both traders must reach the same point B.

Figure 50

98
Because B is optimal for trader S, his indifference curve is tangent to the budget
constraint. Similarly because B is optimal for trader J, his indifference curve is
tangent to the budget constraint. Noticing that the budget constraint is the same
line because it passes through A and it has slope –p
x
/p
y
, it follows that the two
indifference curves (for S and J) are tangent to the same line and therefore
tangent to each other. Since the two indifference curves are tangent to each
other at B, there is no possibility of mutual improvement starting from B (no
lens). Therefore B is Pareto optimal.
74. Disequilibrium. For different prices, consumers may pick different
points. For example, in Figure 51, the budget line is drawn with a different
slope. Sandy picks B and Jamie picks C. The arrows indicate the trades that
Jamie and Sandy are willing to make. Sandy is willing to give up more x
(lower horizontal arrow) than Jamie is willing to buy (upper horizontal arrow).
Therefore there will be leftover x after trade. But Sandy wants more y (left
vertical arrow) than Jamie is willing to give up (right vertical arrow).

99

Figure 51
Therefore there will be more demand for y than the total supply available.
demand of y = y
C
J
+ y
B
S
> y
A
J
+ y
A
S
= supply of y

This is a disequilibrium situation. In Figure 52, the excess demand for y is
seen as a vertical double arrow on the far right.